In process control applications in which the activity or concentration of cationic species such as sodium, potassium or ammonium ions must be carefully monitored, ion selective electrodes, such as glass electrodes, provide a convenient means for providing the desired information. In electrical power generating stations, for example, electrodes that are sensitive to sodium ion concentration can provide a continuous indication of the effectiveness of the ion exchange beds through which the boiler feed water is passed prior to being supplied to the boilers. In such use, an increase in the electrode output reading from a value on the order of one part per billion to a value of several parts per billion may be used as an indication that the ion exchange capability of the ion exchange bed is approaching exhaustion. This allows the ion exchange bed to be removed from service for regeneration prior to the time that increased levels of dissolved salts can damage the interior of the boiler system.
One serious limitation on the usefulness of ion selective electrodes is the limited ability thereof to discriminate between the cation of interest and other small cations. Sodium ion sensitive electrodes, for example, are known to respond to some degree to ammonium and potassium ions. Particularly troublesome, however, is the sensitivity of sodium ion sensitive electrodes to hydrogen ions. This is not only because of the greater interferent effect of hydrogen ions, but also because hydrogen ion concentration bears no meaningful relationship to the total dissolved salt content. As a result, in the absence of precautions for eliminating the effect of hydrogen ions, the output reading of a sodium ion sensitive electrode could be substantially greater than the actual concentration of sodium ions, causing unnecessary servicing of ion exchange beds.
Prior to the present invention, the above-described problem has been dealt with in one of two ways. One of these ways has involved the addition of compounds that increased the alkalinity of the sample. An example of this approach is shown and described in U.S. Pat. No. 3,941,665, issued on Mar. 2, 1976 in the name of Eckfeldt et al.
Another approach to solving the problem of cationic interference involves the division of the sample stream into first and second parts, each of which is applied to a respective ion selective electrode. By passing one of the parts through an ion exchanger of known effectiveness in removing the cation of interest, the ion selective electrode that is exposed to that part in effect provides a signal indicative of the concentration of interfering cations. By then taking the difference between the output signals of the two ion selective electrodes, the effect of interfering cations is made to cancel, leaving an interference free signal that is proportional to the concentration of the cation of interest. An example of this approach is shown and described in U.S. Pat. No. 3,839,162, issued on Oct. 1, 1974 in the name of Ammer.
The problem with the first of the above-described solutions to the interference problem is that the addition of pH raising compounds such as methylamines chemically contaminates the sample. This problem is particularly severe in applications in which the sample liquid flows in a closed system. The feed water of a power generating boiler, for example, may be vaporized, condensed and recirculated many times during its useful life. As a result, any compounds that are added to the flow will, over a period of time, accumulate to undesirable levels. Another problem with the use of such compounds is that they are toxic to humans. Such problems, of course, are in addition to the costs of providing the compound and the cost of the apparatus for delivering the same.
The problem with the second of the above-described solutions to the interference problem is the high cost of providing and maintaining the ion exchange beds through which the sample is processed. Another disadvantage of this approach is that it creates a risk of serious errors, since there is no effective way of determining when the ion exchange beds cease to effectively remove the ions to which they are targeted. As a result, the user is faced with the choice of regenerating the ion exchangers on a precautionary basis, i.e., while they are still usable, or waiting until the beds are known to be exhausted and thereby risking the failures that may occur before the exhaustion of the ion exchanger is discovered.